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(Title of the Thesis)* INVESTIGATIONS INTO ENANTIOSELECTIVE BORO–ION CATALYSIS by Brian P. Bestvater A thesis submitted to the Department of Chemistry In conformity with the requirements for the degree of Master of Science Queen’s University Kingston, Ontario, Canada (March, 2017) Copyright ©Brian P. Bestvater, 2017 Abstract 1,2,3-Triazolylidene carbene stabilized borenium ions were investigated as frustrated Lewis pair (FLP) type hydrogenation catalysts and were found to be able to effect the hydrogenation of imines and N-heterocycles at ambient and near ambient pressures and temperatures. The unprecedented activity of our MIC-borenium ion catalysts was attributed to the high σ-donating ability of 1,2,3-Triazolylidene-based carbenes producing an exceptionally stable and active borenium ion catalyst. Several modifications to the original catalyst structure were employed in an attempt to develop an enantioselective variant of our MIC-borenium ion catalyst. One of the modification to impart asymmetry into the reaction manifold that was investigated in this thesis was the use of a chiral borane, specifically the 9-borobicyclo[3.3.2]decanes (BBDs), in place of the achiral 9-BBN present in the original catalyst design. Disappointingly catalyst stability proved to be highly problematic leading to a difficulty in synthesizing the necessary carbene-borane and resulting in a highly unstable and ineffective catalyst. During the course of our studies into enantioselective borenium ion catalyzed hydrogenations we became concerned with the facility of hydride abstraction by MIC-borenium ions and whether or not this would result in the racemization of enantioenriched α-N sp3 C–H centres. NMR studies revealed that our MIC-borenium ion catalyst was not able to abstract hydrides from α-N sp3 C–H’s at ambient temperatures and only appreciable rates of hydride abstraction were observed at elevated temperatures of 60 °C. This led us to conclude that the reversibility of the hydride addition could not account for poor enantioselectivity observed for chiral borenium ion catalyzed hydrogenations and showed that an enantioselective MIC-borenium ion catalysts should be possible. The MIC-borenium ions were also investigated for their ability to activate Si–H bonds, and if they could be used to catalyze the hydrosilylation of C=N functionalities. The MIC-borenium ii ions were found to be highly active hydrosilylation catalysts, able to tolerate a variety of functional groups and able to effect the rapid reduction of imines under ambient conditions. Finally BBD based borohydrides were also investigated as enantioselective nucleophilic hydrosilylation catalysts. Although they were able to effect the hydrosilylation of ketones, the BBD’s were only able to impart minor enantioselectivity into the catalytic manifold. iii Co-Authorship I hereby declare that this thesis incorporates material that is the result of a joint research project as follows: This thesis incorporates the outcome of research undertaken with the assistance of visiting student Ryoto Kojima and postdoctoral research fellow Dr. Patrick Eisenberger. Unless otherwise stated the experimental efforts and data analysis were performed by the author. I certify that, with the above qualification, this thesis and the research to which it encompasses is the product of my own work. iv Acknowledgements There are many people I need to thank for their contributions to the work presented here, however the greatest debt of gratitude I owe is to my parents, Doug and Valerie Bestvater to whom I dedicate this document. Without your ceaseless and unconditional support, patience, and love I could not have completed all my years of schooling let alone scale the colossal mountain that this document represents. Thank you for all you have given me. Amongst the many others to whom I owe my thanks for their guidance during my time here at Queen’s I would first like to thank my supervisor Dr. C. Crudden. Thank you for giving me the opportunity to work in your laboratory, pushing me to accomplish more than I ever thought I could. I have learned a great deal since I began my studies under your guidance. You have given me the opportunity to meet many brilliant scientists, not least amongst them is yourself, and for that I am thankful. I would also like to thank Dr. Patrick Eisenberger for his guidance and friendship these past years. Patrick, I am still in awe of your mastery of chemistry, both in the laboratory and in your truly awesome command of the literature. Thank you for all you have taught me, allowing me to work with you on your projects, and for all the late night beer sessions despite your insistence on the most revolting, hoppiest IPA in whatever bar we were patronizing. Lastly I would like to thank the remainder of the Crudden group. From my first days here you guys welcomed me in, everyone was always ready to offer their help in the lab, we had great discussions during our weekly synthesis meetings and our group parties were always something the rest of the department envied. Megan, you are the glue that holds this group together and your assistance and friendship has been very much appreciated these past years. I would like to thank Chris Ziebenaus, Eric Keske and Sam Voth. There isn’t much I can say except you guys are great, each of you has taught me so much and we had some great times, though they are quite hazy thanks to Sam. I would also like to thank Jason Rygus and Josh Clarke, again both of you guys have been v great friends and I wish you both the best of luck in your future endeavors as well as the rest of the Crudden Group. vi Table of Contents Abstract ............................................................................................................................................ ii Co-Authorship ................................................................................................................................ iv Acknowledgements .......................................................................................................................... v List of Figures ................................................................................................................................. ix List of Tables ................................................................................................................................ xiii List of Schemes ............................................................................................................................. xiv List of Abbreviations ..................................................................................................................... xv Chapter 1 Introduction; Lewis Acids and Bases, Boro-cations and their Application to Main- Group Catalysis ................................................................................................................................ 1 1.1 Introduction ............................................................................................................................ 1 1.2 Lewis Acids and Bases .......................................................................................................... 2 1.2.1 Lewis Acids as Catalysts ................................................................................................ 3 1.2.2 Lewis Bases as Catalysts ................................................................................................ 6 1.2.3 Lewis Acids-Base Pairs – Classical Adducts and Steric Frustration ............................ 11 1.3 Hydrogenation in Organic Chemistry .................................................................................. 11 1.3.1 Transition-Metal Free Hydrogenations and Frustrated Lewis Pairs ............................. 13 1.4 Hydrosilylation .................................................................................................................... 24 1.4.1 Metal-Free Hydrosilylations ......................................................................................... 27 1.5 Boro–Cations ....................................................................................................................... 29 1.5.1 Boron Cations as Catalysts ........................................................................................... 31 1.6 Research Objectives ............................................................................................................. 37 1.7 Thesis Objectives ................................................................................................................. 38 1.8 Chapter 1 References ........................................................................................................... 39 Chapter 2 Investigations into Boro-Ion Catalysis .......................................................................... 54 2.1 Mesoionic Carbene-Borenium Ion Catalyzed Hydrogenations ........................................... 54 2.1.1 Introduction ................................................................................................................... 54 2.1.2 Non-selective MIC-Borenium Ion Hydrogenation Results and Discussion ................. 55 2.1.3 Enantioselective MIC-Borenium Ion Hydrogenation Results and Discussion ............. 64 2.1.4 Investigations into Borenium Ion Catalyzed Amine Racemization .............................. 71 2.2 Introduction Boro-Ion Catalyzed Hydrosilylation ............................................................... 74 2.2.1 Mesoionic Carbene-Borenium Ion Catalyzed Hydrosilylation Results and Discussion76
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